It's also my impression that the secondary illustrator made a minor contribution. The second hand is almost like a "touch-up" hand that went through the manuscript and filled in what was missing (or might be misinterpreted).
Whether this was at the time the original drawings were done or later, I'm not sure. It's entirely possible that it happened later. When male-looking nymphs are changed to female, one wonders if the second person was respecting the original intention. And yet it's possible that the secondary hand had more authority than the first, so if it was contemporary, then the second hand might have been making corrections rather than guessing.
Whether this was at the time the original drawings were done or later, I'm not sure. It's entirely possible that it happened later. When male-looking nymphs are changed to female, one wonders if the second person was respecting the original intention. And yet it's possible that the secondary hand had more authority than the first, so if it was contemporary, then the second hand might have been making corrections rather than guessing.